Current wireless communications systems are directed to providing RF coverage and/or call capacity so that users may connect to the wireless infrastructure. All solutions rely on some means of distributing RF energy ranging from high power, large coverage area towers to low power in-building pico-cells.
There also exists a class of RF enhancement technologies known as RF repeaters. Some are bidirectional RF amplifiers that retransmit the signals received over the air from a host base station. Others are directly connected to a host base station and distribute the RF signals via either electrical, e.g., coaxial cable, or optical fiber distribution networks. In many cases the signals from a base station can be distributed to multiple antenna sites with a means called simulcast. Time Division Duplexed (TDD) systems represent a unique challenge due to DAS distribution delays and synchronization with the donor base station. More specifically, Distributed Antenna Systems are used to provide wireless communications coverage where it is impractical to install a conventional base station. An example is in-building coverage where low cost radiating antennas are desired and base stations represent either too large or too expensive a solution. Distributed Antenna Systems allow a donor base station to be located outside the desired coverage area and its RF signals are distributed to multiple antennas using either electrical or optical means. A means to distribute the base station's signals to more than one antenna is termed simulcast. In the direction toward the wireless user, i.e., downlink/forward path, the signal is replicated for each remote location. On the return direction, i.e., uplink/reverse path, the signals from multiple remote locations are summed to create a single composite signal for the base station. For both the base station and the user's device, the multiple copies of the RF signal appears as multipath reflections and is compensated for by the use of equalizers and rake receivers.
Simulcast is readily accomplished with a base station providing RF inputs and outputs. These techniques are well known to those skilled in the art. Also, for digital distribution, antenna remoting techniques are known to those skilled in the art.
TDD base stations provide an added layer of complexity for distributed antenna systems that use active electronics. Since a TDD system shares the same frequency for both downlink and uplink in a time shared fashion, the DAS must very precisely derive TDD timing information to prevent equipment damage to either itself or the host base station, and to prevent jamming of other base stations by the DAS remotes. An example of such a TDD airlink is WiMAX with details widely available in the public domain.
In FIG. 1 a block schematic drawing of a Distributed Antenna System (DAS) having direct RF connection to the donor TDD base station with analog optical distribution to the Remote RF Units is shown. Simulcast distribution may be performed either in the RF or optical domains.
In FIG. 2 a block schematic drawing of a DAS having direct RF connection to the donor TDD base station with digital optical distribution to the Remote RF Units is shown. Simulcast distribution may be performed either in the RF or digital electrical domains.
As shown in FIGS. 1 and 2, the current DAS solutions use either analog, i.e., ‘RF over fiber’/‘Analog DAS’, links or sampled digital, i.e., ‘digital DAS’, links and are based on an analog RF connection to the base station. The DAS signals are fed to one or more RF modules, through a technique called simulcast.
Simulcast is readily accomplished with a base station providing RF inputs and outputs. These techniques are well known to those skilled in the art. Also, for digital distribution, antenna remoting techniques are known to those skilled in the art.
FIGS. 1 and 2 show a single base station sector 102, i.e. group of RF carriers, connected to multiple remote RF Units 110. This is not just a demultiplexing operation where multiple RF carriers from the host base station are separated for distribution to separate Remote RF Units. All Remote RF Units transmit and receive the same group of RF carriers as the host/donor base station to which they are connected.
The Remote RF Units are at a different geographical location and they provide either widely separated or partially overlapping coverage areas. For the latter a mobile user's radio may receive identical signals from multiple Remote Units and that composite signal will appear as multipath to that wireless device. For TDD systems, the transmit/receive timing from individual DAS remote units and surrounding non-DAS radiators must also be closely matched, to prevent the transmit cycle of one from jamming the receive cycle of another.
These same multipath and time delay considerations also apply in the reverse direction where a user's device signal is received by multiple remote units. The multiple received signals are summed within the simulcast hardware of the DAS system to provide a single composite signal to the host donor base station 102. Additionally, the TDD base station sets timing constraints on the amount of time delay differential that can be tolerated from the DAS.
For a purely analog distribution network, illustrated in FIG. 1, the simulcast can be accomplished through RF splitters on the downlink, and RF summers on the uplink. The same splitting and summing can be accomplished in the analog optical domain, with the requirement that different optical wavelengths be used on the uplink. A digital distribution network, illustrated in FIG. 2, adds the extra steps of Analog-to-Digital and Digital-to-Analog conversions at both ends of the DAS network. As with the analog DAS, a set of RF summers and splitters can perform simulcast prior to conversion to the digital domain. Simulcast can also be implemented in the digital domain prior to conversion to digitally modulated optical signals.
There is now a new class of base stations with digital input and outputs that are meant to be used in conjunction with remote radio equipment to provide installation flexibility. Although these base stations allow the radio equipment to be remotely located from the base station core electronics, they require a one to one correspondence between each digital airlink stream and a remote radio unit. Detailed specifications of two digital base station interfaces are the Common Public Radio Interface (CPRI) and the Open Base Station Architecture Initiative (OBSAI). With this, a wireless coverage system incorporating a large number of remote antennas will require a large number of base stations along with the attendant issues of frequency re-use and wireless handovers as a user's radio moves throughout a coverage area. These interface standards have also incorporated WIMAX (Worldwide Interoperability for Microwave Access), a TDD airlink, into their specifications.